Metal tanks have been used for years to contain a variety of liquids and gases, in devices as diverse as small personal air tanks for underwater exploration, giant tanks mounted on a truck chassis for transporting industrial chemicals, and squat propane tanks for backyard grills. The end use of the tank determines the materials and tolerances used to make the tank.
For applications requiring superior strength-to-weight characteristics, chemical resistance, and predictable stress characteristics in specific directions, these needs may be met with new techniques and materials, some of which are provided by filament-wound, plastic pressure vessels. Generally, these tanks are reinforced with strands of fibers ("roving"), particularly continuous glass fibers, that are pre-impregnated with a suitable thermosetting resin such as polyester or epoxy resin. The roving is wrapped about a mandrel under controlled tension and in a predetermined pattern. The resin-impregnated rovings wound about the mandrel are often referred to as "windings." The mandrel may be a flexible or disposable material that can be removed after the resin in the windings cures. Alternatively, the mandrel may be a permanent part of the finished tank. In the latter arrangement, the mandrel is a liner and may be blow-molded or rotationally cast out of a thermoplastic which is compatible with the thermosetting resin carried by the windings and with the fluid which the finished tank is intended to contain. Thus the liner provides a fluid-tight and chemical-resistant barrier and the windings provide structural reinforcement.
The unique properties of filament-wound pressure vessels (generally referred to as "tanks" hereinafter) lead to their use in a wide variety of applications, such as pressure accumulator tanks in home water systems and in filter tanks for swimming pools and the like. In such applications, it is desirable, and often necessary, to provide access openings in the end walls and/or side wall of the pressure vessel for fittings such as distributor tubes, plumbing connections, or valve connections, for adding chemicals such as water softeners and chlorine to water in the tank before it is used.
In order to provide fluid-tight connections at access openings in the end walls and side wall of the tank, the surfaces around these openings generally undergo a machining operation to flatten and smooth the molded surfaces, and perhaps also to add a groove for holding a seal, such as an o-ring. The tank is generally held in a fixture for the machining operation. Electronically controlled machine tools, including numerically controlled (NC) machines, have allowed industry to perform very precise machining operations in an automated, consistent and repeatable manner. With the increasing precision in machining operations, industry requires fixtures that consistently locate a workpiece in a very precise manner. If the workpiece is slightly out of position in the fixture, the precision of the machining operation is not fully realized.
To support a generally round workpiece for a machining operation, the workpiece is traditionally supported in a fixture, such as a V-block clamp that supports the body of the workpiece. Such traditional fixtures are not suitable for precision machining a filament-wound tank, however. Although the tank has a generally round shape, the windings give the tank an irregular surface and an unpredictable shape. The irregular surface created by the windings often positions the tank off-center in the fixture, such that differences in windings on various tanks place the centers of different tanks in different positions. Every time a different tank is placed in the fixture, the position of the fixture must be carefully readjusted relative to the machining tool. This makes it difficult to automate the process or to consistently maintain tight tolerances as required by industry.
In addition, since a filament-wound tank generally is formed of a plastic material, as a machining tool presses against the tank, the surface of the tank tends to deflect or move away from the machining tool. The interaction between the machining tool and the flexible tank may also create vibration and chatter in the machining tool and/or the tank, thereby further deterring attempts to consistently maintain tight tolerances. As a result, the machining operation must proceed slowly, with repeated passes over the same surface, and the machining tool must be maintained in a particularly sharp condition. Industry needs a fixture which can precisely and repeatedly locate a tank in the fixture for machining and that can support the tank in such a way that the deflection of the tank under a machining tool is minimized.